Method to measure the angle of incidence for radiation and a detector to carry out the method
BACKGROUND OF THE INVENTION 
     The present invention relates to a method for measuring the angle of incidence for radiation and a detector for carrying out the method. The background for the invention is that in certain electromagnetic radiation, for example laser light of different wavelengths from UV to IR, there occurs an irregularity, speckledness, that is among other things caused by turbulence in the atmosphere. The speckledness varies with a frequency of up to 100 Hz. Experiments that have been carried out have shown that this phenomenon, that means a great variation in the irradiance of the radiation that falls onto a detector, gives rise to problems during the registration of a single or a few short pulses of radiation. On one hand the probability of detection can decrease considerably and on the other hand difficulties of reliably determining the angle to the source of radiation with high resolution can arise. 
     SUMMARY OF THE INVENTION 
     The latter problem is solved by the invention by giving it the design that is evident from the following claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     In the following, the invention will be described in more detail with reference to the accompanying drawing that shows a detector sensitive to the angle of incidence in accordance with the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The basis for the idea of the invention is that one places two detector elements 1,2 with their detector planes in a certain angle to each other and so close to each other that the correlation between the irradiance of the radiation towards them is high. Both detector elements measure towards the same speckle in the speckled radiation. 
     The correlation between the irradiance detected by the detector elements 1,2 depends on the mounting distance between the two detector elements. The wavelength, the illumination distance and the atmospheric condition also play a part here. As an example the irradiances are completely uncorrelated if the detector elements 1,2 are mounted at a distance of more thanabout 5 cm, if the wavelength is 1 μm and the illumination distance is 2000 m. (This is calculated from √λ·L,λ=the wavelength and L= the illumination distance). When the distance is about 4 mm the correlation between the signals from the detector elements are about 0.9, which is a minimum value if one is to get useful results by the invention. The example is valid for a moderately turbulent atmosphere. 
     As one tries to get a small average distance between the two detector elements 1,2 it can be suitable to use rectangular or in another way oblong detector elements that are placed with the long sides against each other. From the point of view of correlation it is better the closer the elements are placed. Experiments have been carried out at 1 μm with detector elements having the measures 0.6×2.5 mm, which for reasons of manufacturing technique were placed with the long sides about 1.4 mm apart. 
     The detector elements 1,2 can be of a completely conventional type, chosen according to the wave length and pulse length of the radiation to be detected. The signal caused by the radiation from each detector element depends on the angle of incidence of the radiation. Each detector has a sensitivity beam, within which the sensitivity for incident radiation varies in a way specific for each detector type. With knowledge of the shape of the sensitivity beams for the detector elements used, the direction of the incident radiation can be referred to a desired reference direction by an angle α by means of a suitable signal processing. If, for example, one uses a conventional detector of the type that has a sensitivity beam having essentially a cosinusoidal dependence of the angle of incidence, the signal S 1 ,S 2  from each detector element arising from the radiation will depend upon the angle of incidence for the radiation according to the cosine law. 
     The cosine law gives: 
     
         S.sub.1 =E cos α 
    
     
         S.sub.2 =E cos (β-α) 
    
     where 
     S 1  =The signal from detector element 1, 
     S 2  =The signal from detector element 2, 
     E=the power density (W/m 2 ) of the incident radiation 
     α=the angle sought for =the angle with the normal of the detector plane 1, 
     β=the known angle between the detector planes. If one solves with respect to α one gets ##EQU1## Thus it is easy to calculate the angle of incidence for the radiation in relation to a given reference direction.